I believe the main problem with the Civ5 AI is that it doesn't understand how to judge the overall combat situation effectively. The AI can usually judge when it can handle a war, declares war and then sends units in the direction of the opponent. It can handle small combat situations pretty well, meaning it does the combat calculations well based on what knowledge it has, but that's where the functionality ends. Once the units get close to my boarders I usually split its forces by targeting it's ranged attackers from the flanks and it is game over damn near every time. It doesn't understand how to amass forces for an attack properly, how to keep the initiative or use tiles in defensive positions to make counterattacks. Also it makes no plans for combat unit relief after it has committed its initial forces.